The present applicant discloses, in Patent Documents 1, 2, and so on, a means which captures a vibration waveform with around 1 Hz reflecting an autonomic nervous function (hereinafter, referred to as an aortic pulse wave (APW)), based on vibration generated on the body surface of the back, finds a frequency time series waveform from a time series waveform of the vibration, and further finds a time series waveform of frequency gradient and a time series waveform of frequency fluctuation, and analyzes their frequencies to determine a state of the person. This means is capable of detecting the aortic pulse wave (APW) in an unconstrained state, and thus, when applied to a vehicle seat, it is excellent as a means for obtaining bio-information of the driver while he/she is driving, and is capable of detecting a hypnagogic symptom phenomenon, an imminent sleep phenomenon, and so on of the driver.
Since the aforesaid APW, heartbeat, or the like is a biosignal reflecting the autonomic nervous function, analyzing this enables the determination of the state of the person as described above, but these biosignals reflecting the autonomic nervous function have very low frequencies of several Hz or lower, for example, belonging to a range from the ULF band (ultra low frequency band) to the VLF band (very low frequency band), and are likely to be buried in external vibration inputted from the floor of the vehicle. So, in extracting the biosignal, an influence of the external vibration has to be eliminated by, for example, the use of the aforesaid time series waveform of frequency gradient. Even if the biosignal can be extracted using the time series waveform of frequency gradient and the like, output signal data detected from a sensor is desirably more suitable for extracting the biosignal.
In consideration of the above, the present applicant proposes, in Patent Document 3, an apparatus which amplifies a biosignal by using string vibration of a three-dimensional knitted fabric disposed as a biosignal detection sensor and disposed in a hole portion formed in a bead foam and also by co-using membrane vibration of a bead foam stacked on one surface or both surfaces of the three-dimensional knitted fabric, to thereby detect the biosignal by a vibration sensor with as high sensitivity as possible.